As this conventional type of ultrasonic sensor, there are examples disclosed in Patent Document 1 to Patent Document 3, for instance. An example disclosed in Patent Document 1 has a structure such that a recess (concavity) is prepared in the back face of a bumper, and that one face of a ceramic 8 for generating an ultrasonic wave that is not come into contact with a metal base 9 (hereinafter, referred to as “surface of ceramic”) is pressed against the face of the recess on the back side of the bumper by the metal base 9 and a pressure cover 27, or a pressure leaf spring 34.
An example disclosed in Patent Document 2 has a structure such that when a recess is formed in a bumper, a vibrating face of an ultrasonic element is directly brought into intimate contact therewith to be stuck and fixed by adhesive. Also, an example disclosed in Patent Document 3 has a structure such that a projection is provided on the bottom (transmitting surface) of an ultrasonic sensor, and that the projection is brought in contact with a mounting member.